Kerry Kennedy was 'sleep driving' according to her defense attorney at DWI trial in Westchester County

Kerry Kennedy leaves a White Plains Courthouse with her mother, Ethel Kennedy, and brother Douglas Kennedy on the first day of her drugged-driving trial. (Richard Harbus for New York Daily News)

Kerry Kennedy took the "sleep driving" defense out for a spin on Monday.

The strategy was unveiled as the drugged-driving trial of the daughter of assassinated Sen. Robert F. Kennedy got underway.

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"This case is about a mistake, plain and simple," defense attorney Gerald Lefcourt said in Westchester County Court.

Kerry Kennedy is charged with impaired driving in a July 2012 crash. (Stephen Lovekin/Getty Images)

"She certainly didn't know at any time while driving that she had accidentally taken the zolpidem, because the evidence will show that this person, Kerry Kennedy, would in a heartbeat, if she thought anything was wrong, pull over."

Zolpidem is a sleep medication better known as Ambien.

Kerry Kennedy's defense is that she mistakenly took a sleeping medication instead of her thyroid pill on the day of the crash. (JB Reed/Bloomberg)

Lefcourt said Kennedy mistakenly took the sleeping pill instead of her thyroid medication on July 13, 2012, before she swerved her Lexus SUV into a tractor-trailer on Interstate 684.

Nobody was hurt, and Kennedy was found still inside her damaged vehicle after the collision.

Kerry Kennedy arrives in Westchester County Courthouse to attend her drugged-driving trial following a hit-and-run in Westchester. (Richard Harbus for New York Daily News)

Lefcourt said the fact that his client, the ex-wife of Gov. Cuomo, is a Kennedy has no bearing on the case. "She is not seeking any advantage here because of her famous family," he said.

But when Lefcourt began describing Kennedy to the four men and two women on the jury as a devoted mother of three and a churchgoing do-gooder, Supreme Court Justice Robert Neary interrupted the halo burnishing and told him to get to the point.

Ethel Kennedy, left, enters the courthouse in White Plains to show support for her daughter. (Richard Harbus for New York Daily News)

"The opening statement is to address the evidence in the case," he said. "We understand the background."

Kennedy, 54, is expected to testify later this week.

Kerry Kennedy, who serves as president of RFK Center for Justice & Human Rights, is unlikely to serve hard time if convicted since she has no prior record. (Richard Harbus for New York Daily News)

Assistant District Attorney Stefanie DeNise tried to pour cold water on the defense.

"She would have known she was not in the right condition to drive" before she took off for the gym, said DeNise.

Witness Henry Myers testified he saw Kennedy's SUV slam into the truck and blow out a tire.

Another witness, William Carlino, told the court that when he pulled behind the SUV he "noticed that the driver's head was slumped against the steering wheel."

Carlino was followed by testimony from two cops and a state trooper, who said Kennedy "seemed very disoriented."

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Dressed in a gray skirt suit, black heels and glasses perched atop her shoulder-length blond hair, Kennedy took notes during the testimony. She has pleaded not guilty. If convicted, she faces up to a year in jail. But she's not likely to do time because she has no police record.

Kennedy initially claimed that a seizure caused the accident, but testing showed she had sleeping medication in her system.

Her mother, 85-year-old Ethel Kennedy, was at the trial to show support. So were two of her brothers, Robert Jr. and Douglas.